1. Field of the Invention
The invention relates to a method and apparatus for determining the point at which a pipe is stuck in a borehole and, more particularly, to a system for magnetically determining a pipe's free point location without the necessity of attaching apparatus to the pipe wall.
2. History of the Prior Art
In the drilling of oil and gas wells through earth formations it often occurs that the drilling pipe will become stuck in the borehole being formed. This may happen because of a collapse or cave-in of the subterranean formation surrounding the borehole. It may also occur as a result of fluid absorption and swelling of certain downhole formations which restrict the movement of the drilling pipe within the borehole, as well as for many other reasons. When this phenomenon does occur, the drilling pipe becomes jammed, operation ceases and no further progress can be made in deepening of the borehole until the stuck pipe is removed.
The first step in clearing a jammed pipe in a borehole is locating the point along the borehole, often several thousand feet beneath the surface, at which the pipe is stuck. Numerous techniques have been developed over the years for locating the free point of the pipe in the borehole so that the pipe portion above the stuck region can be removed. The most popular technique involves the lowering of a tool down the central passageway of the drilling pipe and the attachment of a pair of relatively movable sensor members to the inside pipe wall. The drill pipe is then stretched either longitudinally or in torsion so that any relative movement between the two fixed members indicates that the members are fixed to the pipe wall at a location above the stuck point. Of course, stresses in the drill string which are induced from the surface are only reflected in that portion of the drill string which is above the stuck point. As soon as the sensor pair is affixed to the walls of the pipe below the stuck point and the drill string is stressed, there will be no relative movement between the two members. Thus, by sequential measurement and movement of the sensors along the inside of the drill pipe, the stuck point is located. Systems of this type are, however, relatively slow in that the sequential attachment and detachment of the sensor members requires time and time in the operation of a drilling rig is very expensive. In addition, the contacting type of stuck pipe detectors also require elaborate mechanical or magnetic means for attaching the sensor members to the wall of the pipe.
A known characteristic of ferromagnetic pipe is that the magnetic permeability of the material changes as a function of stresses in the material. Another prior art stuck point detector system has utilized this principle rather than the mechanical elongation of the pipe. Employment of this technique allows the construction and use of a non-contacting stuck point detector which does not need to engage the sidewalls of the pipe. As shown in U.S. Pat. No. 2,686,039 to Bender, a high frequency oscillator 10 is tuned to a frequency on the order of 20 to 50 KH.sub.z by a coil 12 and lowered into the axial bore of a stuck drill pipe. The coil is inductively coupled to the wall of the steel pipe which loads the coil and is thus a part of the tuned tank circuit of the oscillator 10. The magnetic permeability of the pipe determines the degree of loading of the coil 12, therefore, the inductance of the tank circuit and the frequency of the oscillator. As the coil passes the stuck point of a drill pipe under stress, the oscillator will shift in frequency due to the fact that the magnetic permeability of the unstressed pipe below the point is different from that of the stressed pipe above the stuck point. While the Bender system is capable of detecting the stuck point without physical attachment of sensors to the pipe walls such a system includes a number of inherent disadvantages. Perhaps the greatest of these is that the inductive coupling of the pipe into an oscillator tank circuit requires the use of relatively high frequencies. The depth of penetration of high frequency electromagnetic waves are limited by skin effect and thus, the overall accuracy and reliability of the technique is limited. The sensitivity of the Bender system is also restricted by the teaching of a single logging run to detect stuck point which does not allow sufficient tolerance for magnetic permeability variance between different pipe materials and sizes.
While certain other prior art tools have included means for measuring the permeability of pipe or tubing, these are generally utilized only in caliper tools for determining thicknesses and inside diameter of unstressed pipe. For example, U.K. Pat. application No. 2,037,439 of Schlumberger Limited, and U.S. Pat. No. 2,992,390 issued to DeWitte both utilize various aspects of magnetic permeability for pipe measurements.
For example, in the Schlumberger U.K. application, there is described a tool for measuring the wall thickness of a well casing by means of magnetic flux. Three pairs of transmitter and receiver coils are employed, one for measuring inside diameter, one for measuring casing thickness and one for measuring casing wall permeability. Variations in each of these parameters affect one another so that measurements of all three simultaneously can be used to correct one another and produce a highly accurate thickness measurement. While the Schlumberger U.K. application discloses a two coil, two log approach to magnetic permeability measurement, it is only disclosed in connection with a caliper tool and none of these proposals have culminated in a commercially satisfactory stuck point detector.
Although the prior art is replete with both method and apparatus for downhole measurement of pipe permeability, the problem of accurately locating stuck pipe in a borehole has still existed. The system of the present invention has overcome the disadvantages of the prior art to produce a highly successful tool by providing a non-contacting magnetic stuck point detector which uses relatively low frequency to detect changes in permeability occurring in stressed pipe within a borehole. In this manner, an effective system and method is provided for locating the point along the borehole at which a drill pipe section is lodged.